Data-Oriented Design

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Data-Oriented Design (Or Why You Might Be Shooting Yourself in The Foot With OOP) Picture this: Toward the end of the development cycle, your game crawls, but you don’t see any obvious hotspots in the profiler. The culprit? Random memory access patterns and constant cache misses. In an attempt to improve performance, you try to parallelize parts of the code, but it takes heroic efforts, and, in the end, you barely get much of a speed-up due to all the synchronization you had to add. To top it off, the code is so complex that fixing bugs creates more problems, and the thought of adding new features is discarded right away. Sound familiar?That scenario pretty accurately describes almost every game I’ve been involved with for the last 10 years. The reasons aren’t the programming languages we’re using, nor the development tools, nor even a lack of discipline. In my experience, it’s object- oriented programming (OOP) and the culture that surrounds it that is in large part to blame for those problems. OOP could be hindering your project rather than helping it!It’s All About DataOOP is so ingrained in the current game development culture that it’s hard to think beyond objects when thinking about a game. After all, we’ve been creating classes representing vehicles, players, and state machines for many years. What are the alternatives? Procedural programming? Functional languages? Exotic programming languages?Data-oriented design is a different way to approach program design that addresses all these problems. Procedural programming focuses on procedure calls as its main element, and OOP deals primarily with objects. Notice that the main focus of both approaches is code: plain procedures (or functions) in one case, and grouped code associated with some internal state in the other. Data-oriented design shifts the perspective of programming from objects to the data itself: The type of the data, how it is laid out in memory, and how it will be read and processed in the game.Programming, by definition, is about transforming data: It’s the act of creating a sequence of machine instructions describing how to process the input data and create some specific output data. A game is nothing more than a program that works at interactive rates, so wouldn’t it make sense for us to concentrate primarily on that data instead of on the code that manipulates it?I’d like to clear up potential confusion and stress that data-oriented design does not imply that something is data- driven. A data-driven game is usually a game that exposes a large amount of functionality outside of code and lets the data determine the behavior of the game. That is an orthogonal concept to data-oriented design, and can be used with any type of programming approach.Ideal DataIf we look at a program from the data point of view, what does the ideal data look like? It depends on the data and how it’s used. In general, the ideal data is in a format that we can use with the least amount of effort. In the best case, the format will be the same we expect as an output, so the processing is limited to just copying that data. Very often, our ideal data layout will be large blocks of contiguous, homogeneous data that we can process sequentially. In any case, the goal is to minimize the amount of transformations, and whenever possible, you should bake your data into this ideal format offline, during your asset-building process.Because data-oriented design puts data first and foremost, we can architect our whole program around the ideal data format.